At present, liquid crystal displays (LCDs) are the most common type of displays used in products such as notebook computers, game centers, and the like.
The principal driving devices for an LCD are thin film transistors (TFTs). Because the amorphous silicon layer in amorphous silicon TFTs can be made at a relatively low temperature (between 200° C. and 300° C.), amorphous silicon TFTs are frequently used in LCDs. However, the electron mobility of amorphous silicon is lower than 1 cm2/V.S. (one square centimeter per volt second). Hence, amorphous silicon TFTs cannot provide the speeds required of an LCD in certain high-speed devices. On the other hand, the polycrystalline silicon (or polysilicon) TFT has electron mobility as high as 200 cm2/V.S. Therefore polysilicon TFTs are more suitable for high-speed operations. However, the process of transforming an amorphous silicon layer into a polysilicon layer often requires an annealing temperature in excess of 600° C. Under that temperature, the glass substrate supporting the TFTs is liable to be distorted. Thus, a number of methods of fabricating a polysilicon layer at a reduced temperature have been developed. Among such methods, the excimer laser annealing (ELA) method is the most prominent.
In a typical ELA process of fabricating a polysilicon layer, an excimer laser generator generates a cylindroid (cylinder-like) excimer laser beam. The laser beam irradiates an amorphous silicon layer, moving up and down to melt a first area of the amorphous silicon layer. Then the excimer laser generator is stepped a distance, and moves up and down to melt a second area of the amorphous silicon layer that partially overlaps the first area of the melted amorphous silicon layer. Because the temperature of the overlapping portion of the second area is higher than the temperature of the non-overlapping portion of the second area, a lateral temperature gradient exists along the direction from the higher temperature to the lower temperature. Hence, heterogeneous nucleation occurs at the interface of the overlapping portion and the non-overlapping portion to form a few seeds of crystallization. Thereafter, the melted silicon starts crystallizing from the seeds of crystallization to finally form a polysilicon layer. Because the temperature of the ELA process is under 500° C., the polysilicon thin film transistors fabricated using such low temperature process are often called low temperature polysilicon thin film transistors (LTPS-TFTs).
However, in order to melt each area of the amorphous silicon layer, the laser generator has to move up and down several times. This contributes to the duration of the production cycle. Moreover, the size of polysilicon grains of the amorphous silicon layer has a positive correlation to a value of the lateral temperature gradient when the energy of the amorphous silicon layer is under the super lateral growth (SLG) point. In the aforementioned ELA process, the lateral temperature gradient has a relatively low value. Therefore the sizes of the fabricated polysilicon grains are small, and the polysilicon layer has relatively low electron mobility. Furthermore, it is hard to control the energy provided to the amorphous silicon layer. If the energy exceeds the SLG point, a density distribution of the seeds of crystallization may drop to a very low value within a transient interval. The sudden loss of seeds of crystallization may lead to the production of numerous small and highly non-uniform grains. Overall, the typical ELA method of fabricating a polysilicon layer is liable to have low efficiency, and the polysilicon layer fabricated by such method may have relatively low electron mobility.
Accordingly, what is needed is a method for fabricating a polysilicon layer that can overcome the above-described deficiencies.